1. Field of the Invention
The present invention relates to dielectric filters including dielectric blocks having inner conductors formed therein and outer conductors formed thereon, dielectric duplexers, and communication apparatuses incorporating the same.
2. Description of the Related Art
A conventional dielectric filter using a dielectric block is shown in each of FIGS. 9A and 9B. FIG. 9A shows a perspective view of the dielectric filter and FIG. 9B shows a view of the open face side of inner conductors. In each of FIGS. 9A and 9B, the reference numeral 1 denotes a rectangular parallelepiped dielectric block. Inside the dielectric block 1, there are arranged inner-conductor-formed holes 2a and 2b in which inner conductors are formed on the inner surfaces thereof. On a surface of the dielectric block 1 at open ends holes 2a and 2b, there are formed coupling electrodes 3a and 3b connected to the inner conductors. An outer conductor 4 is formed on the remaining five surfaces of the dielectric block 1.
With the above arrangement, there are provided two resonators formed in the dielectric block. The two resonators are coupled via a capacitance generated between the coupling electrodes 3a and 3b. 
In order to broaden the pass band of a band pass filter having a plurality of resonators formed in a dielectric block, the coupling strength between the resonators needs to be increased. As shown in FIGS. 9A and 9B, in the conventional dielectric filter, the coupling electrodes are disposed on the end face of the dielectric block at the open ends of the inner conductors. In order to increase the coupling strength between the resonators, the gap g between the coupling electrodes 3a and 3b needs to be narrowed. On the other hand, on the end face of the dielectric block, where the open ends of the inner-conductor-formed holes are formed, when determining the gap between the coupling electrodes connected to the adjacent inner conductors, even with the use of the narrowest gap obtainable with the accuracy range available for forming electrode patterns, there is a limitation to the amount of capacitance that can be generated between the coupling electrodes 3a and 3b. 
Thus, as shown in FIG. 9C, by arranging mutually opposing portions of the coupling electrodes 3a and 3b in comb-like forms, a relatively large capacitance can be generated in the limited area. However, in order to make such electrode patterns, the electrode-pattern forming method requires high accuracy. As a result, it is difficult to obtain a dielectric filter having good characteristics. Thus, this causes reduced yield rates and an increase in cost.
Furthermore, with the demand for miniaturized communication apparatuses incorporating dielectric filters using such dielectric blocks, the heights of the components used are reduced, so the length of the space (indicated by the symbol h in the figure) between the coupling electrodes cannot be increased. Consequently, since the magnitude of an obtained coupling strength is limited, it is difficult to produce a dielectric filter having a desired bandwidth. In other words, height reduction is eventually limited due to conditions for the coupling strength between resonators to be coupled.
Accordingly, the present invention provides a dielectric filter capable of easily obtaining desired filter characteristics by strongly coupling adjacent resonators with high accuracy while reducing the height of the entire filter. In addition, the invention provides a dielectric duplexer and a communication apparatus incorporating the filter or the duplexer.
According to a first aspect of the present invention, there is provided a dielectric filter including a substantially rectangular parallelepiped dielectric block. The coupling electrodes are having a plurality of inner-conductor-formed holes arranged thereinside. There are inner conductors disposed on the inner surfaces of the holes. In addition, the filter includes coupling electrodes formed on an outer surface of the dielectric block. The coupling electrodes are extended either to a first edge of the dielectric block at which a surface which contains open ends of the inner-conductor-formed holes joins a side surface parallel to a direction in which the holes are aligned, or onto said side surface across said first edge. The coupling electrodes are connected to the inner conductors. An outer conductor is arranged on outer surfaces of the dielectric block. With this arrangement, a large capacitance can be generated between the coupling electrodes.
Furthermore, this filter may further include input/output electrodes arranged on a side surface opposing the first mentioned side surface from a second edge opposing the first edge, to generate capacitances between the open end portions of the inner conductors and the input/output electrodes. With this arrangement, in the state in which the input/output electrodes are connected to electrodes on a mounting circuit board, the coupling electrodes are positioned on the upper surface of the dielectric block so that the electrode patterns do not influence the coupling strength between the resonators inside the dielectric block.
According to a second aspect of the invention, there is provided a dielectric duplexer including the input/output electrodes of the dielectric filter according to the first aspect. The input/output electrodes are used as a transmission-signal input electrode, a reception-signal output electrode, and an antenna-connecting electrode.
Additionally, according to a third aspect of the invention, there is provided a communication apparatus including one of the dielectric filter and the dielectric duplexer. For example, the dielectric filter or the dielectric duplexer is incorporated in a filter circuit for filtering transmission signals and reception signals in a high frequency circuit section.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.